Figure 2 is based on the analysis of Cox et al. (2004), but informed by the results of Buck and Goffinet (2000), Goffinet et al. 2001, Tsubota et al. 2002, and Shaw and Renzaglia 2004. The system implied by the structure of the cladogram is different from those proposed before 1995 (e.g. Schofield 1985). Some of the more recent conclusions are:

• Takakia (Clade 4) is a moss not a liverwort (Buck and Goffinet 2000) and is associated with Sphagnum (Clade 3) as well as taxa like Andreaea (Clade 6) whose capsules dehisce along longitudinal lines (Shaw and Renzaglia 2004).
• The possible associations of groups that contain Polytrichum, Tetraphis, and Buxbaumia (Buck and Goffinet 2000; Clades 10, 12, and 14, respectively).
• A confirmation of the monophyly of the Bryopsida (Clade 15), although the taxa within its hierarchy have been rearranged such that there are no clear synapomorphies for some of the taxa. The unifying synapomorphy for the whole group is the occurrence of peristomal teeth that are jointed and made of cell fragments.

THE BASAL MOSSES: SPHAGNOPSIDA AND TAKAKIOPSIDA

Sphagnum (Figure 3) is a relatively common genus that occurs in cool, wet areas across the globe. It tends to acidify the environments in which it occurs, which, together with low biologically-active nitrogen concentrations, leads to depressed microbial production. Thus, it often is able to grow faster than it degrades allowing accumulations of dead Sphagnum to build up as peat. The characteristic structure of the leaf-like photosynthetic branches, a network of large dead hyaline cells surrounded by small photosynthetic cells, allows Sphagnum to take many times its weight in water (Figure 4). The empty hyaline cells also persist when the plant is dead; so, its water-holding ability is barely diminished when it is peat. The peatlands most often associated with Sphagnum are bogs, and some of them cover boreal expanses of Canada, Russia, and Scandinavia. Sphagnum also can overgrow ponds in depressions that are poorly buffered. In many cases, the moss can cover the surface with a sufficiently thick layer that shrubs and small trees can grow on it. I once sampled such a quaking bog in central Pennsylvania. The peat made a floating layer held together by roots that was about a meter thick with nearly three meters of water beneath it.

The Sphagnum plant begins its growth from a spore and produces a spatulate protonema (not a filamentous one). From that emerges the gametophores, of which the photosynthetic branches are the most obvious constituents. Antheridia develop in the axils of the leaves and archegonia develop at the tips of the stems. Following fertilization, gametophyte tissue at the tip of the gametophore begins to bear the developing sporophyte out of the bud-like leafy structure. The elongation of the stem resembles a seta, but it is not one, but a haploid structure with the unfortunate name of pseudopodium (text with tooltip) A leafless branch resembling a seta, bearing the capsule in some species of bryophytes (particularly in the Sphagnopsida and Andreaeopsida). Typically, the setae in these taxa are very short. After the fertilization of the egg, the tissue at the base of the archegonium grows and bears the capsule away from the leafy gametophore. Unfortunately, these structures that really should be called 'pseudosetae' were given the name pseudopodia (a term already in common usage for the cytoplasmic extensions of amoebae and rhizarians). . The sporophyte has a foot, almost no seta, and a capsule with an operculum (Figure 5). When mature, the sporophyte capsules look almost black and contrast well against the green of the gametophores.

Takakia (Figure 6), once considered to be a liverwort, is now believed to be a primitive moss with affinities to Sphagnum (Buck and Goffinet 2000). It is an alpine moss with a disjunct distribution between North America and the Himalayas. Takakia grows as a filamentous film over rocks with short, brittle gametophores that have highly dissected leaves. It is the only group of mosses in which the gametangia (archaegonia and antheridia) are not protected by perichaetial leaves. The sporophyte dehisces along a slit that spirals down the elongate capsule.

ANDRAEOPSIDA AND ANDRAEOBYOPSIDA

These two small classes are very similar. They grow as very small mosses with short gametophores on rocks. They tend to be pigmented (dark brown or red (see Andraea in Figure 7). The capsule dehisces along four longitudinal slits, which makes them look like small paper lanterns and gives them their common name, Lantern Mosses. Typically, they are mosses of alpine areas, particularly in the southern hemisphere. Andraeobryum (Figure 8) occurs in parts of Alaska and western Canada. The primary difference between the two groups is that the sporophyte of Andraea is borne aloft by a pseudopodium. Andraeobryum has no pseudopodium.

THE PERISTOMAL MOSSES

These mosses are characterized by having sporophytes with operculate capsules. After the operculum comes off of the capsule, the opening is surrounded by tooth-like structures, called peristomal teeth, that open and close according to the humidity and allow for the gradual release of spores.

PERITRICHOPSIDA

Polytrichum, also called the “Hair Cap Moss” is one of the most common woodland mosses in the northeastern US (Figure 9). The unbranched leafy gametophore emerges from the prostrate, filamentous protonema and can be quite long (more than 15 cm long for some species). The stem can grow so high, in part, because it contains specialized water conducting tissue (made of cells called hydroids (text with tooltip) Specialized water-carrying cells found in bryophytes. May have been the precursor vascular tissue. ) and solute conducting tissue (whose cells are called leptoids (text with tooltip) Specialized nutrient-carrying cells found in bryophytes. May have been the precursor vascular tissue. ). The lanceolate leaves are whorled (text with tooltip) When plants have more than two leaves per node, they are said to have whorled leaves. on the stem and have longitudinal lamellae (text with tooltip) Thin sheets of cells usually standing perpendicular to the surface of a leaf. that run the length of each leaf. The lamellae take the place of stomates and allow the rapid uptake of water by the leaves. The antheridial gametophores and flattened on the tops while the archegonial gametophores are pointed. Following fertilization of the egg in the archegonium, the sporophyte emerges by rapid growth of the seta. The developing capsule emerges with a cap of gametophyte tissue called the calyptra (text with tooltip) Small sheath of cells, derived from the archegonium, which covers the top of the capsule. . Following spore formation and maturation, the calyptra falls away and the operculum breaks off of the top. The opening is mostly occluded by a swollen end of the columella (text with tooltip) A small column of tissue running up through the center of the capsule. called an epiphragm around whose margin are many small multicellular teeth. During times of low humidity the teeth raise and allow the release of spores from the capsule.

TETRAPHIDOPSIDA

These small mosses form a dense turf where they grow. The gametophores are small (up to 1.5 cm high). The sporophytes, however, sit atop long setae (Figure 10). The peristome is distinctive in that it is made up of four, large, multicellular teeth.

BUXBAUMIOPSIDA

This is the most problematic of the groups. In the taxonomy of Buck and Goffinet (2000), this class contains the genera Buxbaumia and Oedopodium. However, according to the phylogeny of Cox et al. (2004), Oedopodium and Buxbaumia are separated by the Peritrichopsida, making the Buxbaumiopsida polyphyletic.

The gametophyte is small and not distinctive. However, the sporophyte of Buxbaumia has a red seta and a flattened capsule that sits at an angle. Thus, the common name is “Bug on a Stick”.

BRYOPSIDA: MOSSES WITH COMPLEX PERISTOMES

The most speciose group of mosses is the Bryopsida (Figures 12-24). In general the gametophytes are complex. The upright gametophore stems may be branched or unbranched, but they never contain leptoids. The leaves range from oval to lance-shaped, and usually are tipped with a bristle, called an awn. The gametophores may produce the sporophytes at the tips of the gametophores (acrocarpous) or they produce sporophytes on short lateral branches (pleurocarpous). Many of them (e.g. Mnium; Figure 22) look very much like tiny flowering plants. Taxa within the Hypnales (e.g. Hypnum; Figure 24) are remarkable diverse and complex. They look like tiny creeping ferns and many have different types of stems (e.g. creeping and upright or slender and robust).

All members of the bryopsids are distinguished by the peristomal structure of the capsule. Following the release of the operculum, the tissue covering the spores tears along preset lines such that one to three rows of peristomal teeth are formed. These teeth are different from those of Polytrichum and other peristomal mosses in that the bryopsids all have teeth made of cell fragments and all of the teeth are jointed. As in Polytrichum, the bryopsid teeth are hygroscopic and close when humidity is high, but when the air is dry, they open up to expose the spore chamber allowing them to escape.

DIPHYSCIIDAE: Diphyscium (Figure 12) has a very short seta so that the capsule lies surrounded by the perichaetial “leaves”. The peristome is made of two rows; the outer row has 16 teeth. The gametophyte is made up of very small perennial gametophores.

FUNARIIDAE: Typically, these are acrocarpous and grow on soil (terricolous). The peristome usually is made of two rows, but there are some taxa with a single row. Funaria (Figure 13) has a distinctive sporophyte with a long seta and an inclined capsule.

DICRANIDAE: Most mosses that have peristomal teeth have elements of two cells on the outside of the tooth. Thus, they are called diplolepideous. However, a distinguishing feature of this diverse subclass is peristomal teeth whose outer side is made of the elements of a single row of cells and are termed haplolepideous. We have chosen the following six genera to illustrate the diversity of this subclass.

Grimmia (Figure 14) has gametophore shoots that are acrocarpous and usually grow on rocks. The leaves are egg-shaped to linear with long cilia and are tipped with distinctive long awns (text with tooltip) A bristle at the tip of a leaf. , which gives the moss a hairy appearance. The seta varies from long to short. The capsules are spherical to cylindrical and erect. The peristome is single with 16 teeth, usually divided into 2 segments. The calyptra is hooded or conical.

Archidium (Figure 15) has ephermeral gametophore shoots that are small with stems arising from below the perichaetial leaves (leaves which surround the sex organs). In general, the leaves are lance-shaped with a sheathing base and a terminal awn. The sporophytes are very distinctive with a large foot, a very short seta, and a round capsule with no operculum, peristome or columella. The calyptra is small. The spores produced by this genus are the largest of the mosses (up to 130µm).

Seligeria (Figure 16) is a tiny moss that produces a velvet green covering on limestone. It has a relatively large, inclined capsule with a vell-developed peristome. The spores are very small.

Fissidens (Figure 17) has gametophore shoots that are variable in length, some up to 5 cm long. The leaves, which range from oblong to lance-shaped, are two-ranked (in 2 rows on the stem), broad, and tipped with awns. The seta varies from short to long. The capsule is ellipsoid, erect to inclined. The peristome is single with 16 teeth divided into 2 segments each. The calyptra is hooded and conical.

Dicranum (Figure 18) has large gametophore shoots. The leaves vary from thin to broad, each is tipped with an awn that bends to one side giving the plant the appearance of wind-swept grass. The capsule is egg-shaped to cylindrical, often curved and usually erect. The peristome is single with 16 teeth, each divided into 2 segments. The calyptra is hooded and conical.

Pottia (Figure 19) has small acrocarpous gametophores. The leaves are entire, egg-shaped to linear, relatively large, and have an awn. The seta varies from long to short. The capsule is erect or inclined; with or without an operculum; and generally cup-shaped (ranging from egg-shaped to cylindrical). The peristome is single with 16-32 teeth, often spirally twisted and divided into thread-like segments.

BRYIDAE: These plants tend to be pleurocarpous and have peristomes with two rows of teeth that alternate. The innermost row is ciliate. The mosses in this subphylum are quite comples in appearance and can fool the casual observer into thinking that they are very small ferns. We have chosen the follwoing five genera to represent the diversity of this large group.

Leptobryum (Figure 20) is almost ubiquitous. It produces lance-shaped “leaves” and nodding semi-transparent capsules that are pear-shaped.

Hedwigia (Figure 21) typically produces “leaves” with distinctive white-tipped awns. It grows on rocks and has a branching-spreading habit.

Mnium (Figure 22) has large acrocarpous gametophores (up to 10 cm long) that are erect and spreading. Leaves are broad, usually egg-shaped to lance-shaped with a pointed tip, often with an awn. The capsules are cylindrical to spherical and generally inclined or nodding. The peristome is double, each with 16 teeth. The innermost row is a folded membrane which continues as cilia. The calyptra is small, hooded and quickly lost.

Hookeria (Figure 23) grows on wet rocks. Its “leaves” are pale, almost translucent.

Hypnum (Figure 24) represents a large and very complex group of mosses that resemble tiny ferns. The gametophore shoots are pleurocarpous (creeping and erect or slender and robust); up to 5-20 cm long. “Leaves” are curved, lance-shaped, and tipped with awns, which curve in the same direction. Often these plants are found on logs, stones and moist soil. The capsule often is inclined and round, oval or cylindrical. The peristome is double, and the inner teeth are thread-like. The calyptra is hooded.